The invention relates to an insert container for freeze-drying plants comprising a plurality of vertical ribs subdividing the insert container into parallel compartments for the material to be dried.
Such insert containers are being used in continuous and discontinuous industrial freeze-drying of granular materials, e.g. in the production of freeze-dried coffee extract. The frozen and ground material is filled into the insert containers, the latter are hooked into suspension carriages and introduced into the vacuum tunnel. The suspension carriages are transported through the tunnel intermittently and are placed on the installed heater plates so that the bottoms of the insert containers are in direct contact with the heat exchange surfaces.
A certain minimum drying rate is required under an economical aspect. Owing to the resistance offered by the bed of frozen material to the flow of escaping water vapor, there is the risk of an inadmissibly high pressure rise in the bed, which may cause the material to exceed the triple point which, in turn, results in fusion of the product and collapse of the structure. An inadmissibly high pressure rise is avoided by providing for efficient water vapor discharge.
In a process known from German Pat. No. 1,135,831, the material to be dried is frozen in the form of upright profiled bodies of material, and the compartments formed by the ribs are so dimensioned that there remains a sufficient diffusion gap between the ribs and the profiled bodies of material.
From German Auslegeschrift 1,285,954 an insert container is known in which the material to be dried rests on a water vapor-pervious supporting member so that there remains a space between the container bottom and the container walls on the one hand, and the supporting member on the other hand.
The supporting member communicates with the ambient atmosphere through vent holes in the bottom and in the walls of the insert container so that in this way the water vapor can be discharged.
Moreover, it is known for an insert container subdivided into compartments by parallel ribs to hang a V-screen insert into the compartments for discharge of water vapor. The space between the V-screen insert and the ribs serves to receive the material to be dried. The V-screen insert forms in each compartment a vertical narrow pocket communicating with the ambient atmosphere to discharge water vapor.
In the above described methods for discharging water vapor it is a disadvantage that the heat transfer surface for the material to be dried is reduced. Thus, for instance, with the use of a V-screen the heat transfer function of the tray bottom area is largely eliminated.
The heat transfer from the insert container to the V-screen insert, or to a supporting member, respectively, is low due to limited possibilities of contact with the tray bottom. This soon results in soiling of the V-screen insert or of the supporting member or adhesion of material being dried. Such soiling can be removed only by washing. If soiled insert containers were re-used without being cleaned, this would reduce the filling capacity thereof and would drastically impair the drying behavior. Poor and non-uniform drying behavior of food products results in quality losses by partial over-drying, in capacity losses, and in highly non-uniform residual moisture which, in turn, leads to an unnecessarily low average moisture content.
With the use of a V-screen insert or of a supporting member, there is the risk of mechanical deformation with ensuring differences in the rib spacing. The resulting different bed thickness of the material being dried also leads to impairment of the drying behavior and may reduce the filling capacity up to 50 percent. The use of a V-screen leads to very high water vapor flow rates in the region of the upper edge of the V-screen insert, which may result in fluidization of the drying material in said region. As a result, a portion of the drying material is discharged and deposited on the heater plates or in the vacuum tunnel and the condensers. In addition to the losses in material to be dried, there is also a risk of soiling the vacuum tunnel which may impair, for example, also the transfer of heat from the heater plates to the insert containers.
It is the object of the invention to provide an insert container for freeze-drying plants which warrants highly uniform drying of the material.